The present invention relates to strain gauges in general and in particular to a strain gauge plethysmograph comprising an elastic tubular member. The tubular member is filled with a non-mercury, electrically conductive fluid attached to an inelastic strap member. The strap member is provided for fastening the fluid-filled elastic tubular member to an appendage and for restricting the length to which the tubular member can be stretched when fastened to the appendage.
A typical strain gauge plethysmograph, like the present invention, comprises an elastic tubular member filled with an electrically conductive fluid and means for fastening the tubular member to an appendage. The appendage may comprise an arm, a leg, a finger or the like. Coupled to the tubular member, and in electrical contact with the fluid therein, there is provided a resistance-measuring apparatus, such as a resistance bridge or the like. The resistance bridge is connected to the tubular member and the fluid therein as by relatively small-gauge wires for measuring changes in resistance of the fluid due to blood flow in the appendage.
As the tubular member is stretched lengthwise, the cross section of the tubular member and the fluid contained therein is reduced. This reduction in cross section and increase in length result in an increase in the resistance of the fluid. The magnitude of the increased resistance is a function of the magnitude of the reduction in the cross section and increase in length.
When fastened about an appendage, such as an arm, leg, finger or the like, blood flowing through the appendage will affect the size of the appendage, specifically its circumference, resulting in a corresponding change in the length of the tubular member fastened thereabout. Accordingly, as blood flows into an appendage, the appendage increases in size; and, conversely, as the blood flows from the appendage, the appendage typically decreases in size. Using suitable recording apparatus attached to the tubular member, an operator is able to record both the magnitude and rates of change of blood flow in the appendage.
Heretofore, typical strain gauge plethysmographs used for measuring limb blood flow and blood pressure comprised single or folded rubber tubular members filled with mercury which were wrapped about an appendage. In conventional gauges comprising a folded tubular member the ends of the tubular member are secured to a rigid member adjacent to each other forming a loop in the tubular member. When wrapped about an appendage, the mid-point or center of the loop is fitted over and releasably attached to the rigid member to which the ends of the tubular member are attached somewhat in the fashion of a rubber band.
In gauges using a single tubular member, only one wrap of the tubular member is employed. This arrangement is usually found in the smaller digit gauges.
In practice, a number of disadvantages are associated with using mercury-in-rubber strain gauges. One of the disadvantages is associated with the toxicity of mercury. Because of its toxicity, the use and handling of mercury is considered very dangerous and requires costly methods and apparatus to prevent accidental poisoning of persons using or making devices using it.
Another disadvantage of mercury is associated with its characteristic negative meniscus. Because of this characteristic, mercury tends to draw away from a surface with which it is in contact and is found to separate with age. In a strain gauge, this characteristic is a disadvantage because it is important to form a continuous fluid surface or body end to end with respect to the ends of the gauge.
Another disadvantage of prior known strain gauge plethysmographs is associated with unrestricted stretching of the elastic tubing. Unrestricted stretching of the elastic tubing during fastening of the gauge to an appendage is found to result in the application of undesirable pressure on the appendage and to produce artifact induced changes in the output readings of the gauge. For example, unrestricted stretching of the elastic tubing, especially when using mercury having a negative meniscus, results in such artifacts as separation of the mercury in the tube and bubbles along the surface of the tube at points where the mercury has pulled away from the surface of the tube.
Unrestricted stretching of the tubular member also makes it difficult to obtain reproducible pressures on the limb so that any artifacts which may appear by presence of the apparatus will appear differently from one use of the apparatus to another.